Originally published in 1971 as A Guide to Middle-Earth, before the publication of The Silmarillion, the first edition contained only information from The Lord of the Rings and The Hobbit. In 1978, a new edition (The Complete Guide to Middle-earth: from The Hobbit to The Silmarillion), containing material from The Silmarillion, was published.

Structure

A typical entry of the encyclopedia contains: a name; the language it belongs to; its translation from Elvish, Adûnaic and sometimes Old English when known; and known dates when a character flourished; the first lines of the entry usually give a general definition of the topic, like race, heritage and role (in case of a character), leading to a chronological description or biography. The last paragraphs of the entry give a physical description or characteristics of the character with some speculations; the final paragraph gives the topic's alternative names, epithets, translation to other languages, and/or redirects to other entries.

The Guide is generally inclusive and there is no limitation to the topics it covers; it includes even obscure and little explored topics, like individual entries on each single Tengwar names. Many entries are simply epithets and only redirect to the names of their main entries.

Accuracy

Introduction: It is explained that death dates of those who sailed to the West are not given in their characters' entries "for they live still". While this can be true for Gandalf and the Elves, this is also implied for Bilbo, Frodo, Sam and Gimli. This seems to contradict Tolkien's concept that the Undying Lands don't grant immortality[2][3].

Tar-Aldarion: Foster speculates that the tragic relations with his father and wife were because he left no male heirs. The later published text Aldarion and Erendis gives a detailed account on their relationship, mostly owing to Aldarion's obsession with the Sea.

Ambar: Foster relates the Elvish words ambar "world" and umbar "fate". In the entry of Ambar, he mentions it is a concept related to fate of the world. The Etymologies showed that the two have different Roots; a distinct word ambar is actually linked to the root for "home".

Possible Inaccuracies

Bladorthin: Foster supports the usual misunderstanding that Bladorthin's spears were not delivered because he died early; while the text mentions those events closely to imply that they are connected, it doesn't really mention his death as premature or as a reason for the failed delivery.[4]

Buckland: The date Fo.A.42 is mentioned as the date when Buckland and the Westmarch were officially added to the Shire by the gift of King Elessar. There are two mistakes in this statement: 1. The date has been corrected as Fo.A.32 in later editions; 2. Tolkien did not mention that Buckland joined the Shire: in the Prologue a semi-colon is intended to show that the Westmarch was added, but not Buckland.[5]

Gollum: Foster mentions that Déagol was Sméagol's cousin while this is not mentioned in the texts. Tolkien went only as far as to suppose he was "evidently a relative (as no doubt all the members of the small community were)"[7]